This invention relates to magnetic resonance (MR) imaging.
The invention is particularly concerned with reduction in the time needed to collect data for imaging a region of interest of a patient.
A prior art magnetic resonance imaging apparatus is shown in FIG. 1. A patient 1 (shown in section) is slid axially into the bore 2 of a superconducting magnet 3, and the main magnetic field is set up along the axis of the bore, termed by convention the Z-direction. Magnetic field gradients are set up, for example, in the Z-direction, to confine the excitation of magnetic resonant (MR) active nuclei (typically hydrogen protons in water and fat tissue) to a particular slice in the Z-direction e.g. that illustrated in FIG. 1 and, in the horizontal X and the vertical Y-directions as seen in FIG. 1, to encode the resonant MR nuclei in the plane of the slice. An r.f. transmit coil (not shown) applies an excitation pulse to excite the protons to resonance, and an r.f. receive coil arrangement comprising an array of receive coils 4, 5 picks up relaxation signals emitted by the disturbed protons.
To encode/decode received signals in the Y-direction, the signals are detected in the presence of a magnetic field gradient, termed a frequency encode or read-out (R.O.) gradient, to enable different positions of relaxing nuclei to correspond to different precession frequencies of those nuclei about the direction of the main magnetic field due to the influence of the gradient. The data is digitised, and so for each r.f. excitation pulse, a series of digital data points are collected, and these are mapped into a spatial frequency domain known as k-space (FIG. 2). Each r.f. pulse permits at least one column of digital data points to be collected.
To encode/decode the received signals in the X-direction, after each r.f. pulse has been transmitted and before data is collected with the read-out gradient applied, a magnetic field gradient in the X-direction is turned on and off. This is done for a series of magnitudes of magnetic field gradients in the X-direction, one r.f. pulse typically corresponding to a different magnitude of gradient in the X-direction.
On the k-space matrix shown in FIG. 2, the columns of data points correspond to data collected at different magnitudes of phase-encode (P.E.) gradients.
The field of view imaged by the magnetic resonance imaging apparatus depends on the spacing of the data points in the phase-encode and read-out directions, and the resolution of the image depends on how far the points extend in each direction i.e. how large the maximum phase-encode gradient is, and on the magnitude of the read-out gradient combined with the duration of data collection.
Conventionally, the data collected by the r.f. receive coil arrangement and depicted in FIG. 2 is subject to a two dimensional fast Fourier Transform in a Fourier Transform processor (not shown) to produce a pixelated spatial image.
A slice image is shown in FIG. 3. For the purposes of explanation, the symbol of a circle 1a, has been illustrated in both the patient 1 shown in FIG. 1 and the image shown in FIG. 3. FIG. 3 implies that the spacing of data points in the phase-encode gradient direction is sufficient to image the whole of the circle shown in FIG. 1.
Between each r.f. pulse, there is a certain minimum pulse repetition time, and the collection of data implied by FIGS. 2 and 3 may therefore take an undesirably long time.
One technique used to reduce the data collection time is to cut out, say, half the phase-encode steps e.g. by keeping the same maximum phase-encode gradient but omitting every other column of data. This would then halve the data collection time.
The spacing of the datapoints in the phase-encode direction would now have doubled, so that the field of view in the corresponding image domain would have halved. (The field of view in the read-out direction would remain the same because the number of data points collected during read-out would remain the same.) The imaged area would now cover little more than half the width of the circle illustrated in FIG. 1. This is shown by the area 1b in FIG. 5. Unfortunately, aliasing causes the regions at the side of the circle to be folded back into the half-width area, the left hand region in FIG. 5 corresponding to the right hand region of the image, and vice versa.
To enable the data to be unfolded, the data is acquired using parallel imaging.
Parallel imaging makes use of spatial sensitivity differences between individual coils in an array to reduce the gradient encoding required during image acquisition. This reduces acquisition times by decreasing the number of phase-encoded lines of k-space that must be acquired. One practical implementation of parallel imaging, is known as SENSE (Magnetic Resonance in Medicine 42: 952-962 (1999)xe2x80x94SENSE: Sensitivity Encoding for Fast MRI by Klaas P Pruessmann, Markus Weiger, Markus B Scheidegger and Peter Boesiger).
SENSE operates in the image domain for both the target image data and the coil reference data. A typical receive coil arrangement comprises coils 4 and 5 placed on opposite sides of the patient arranged as in FIG. 1, in order that they have different fields of view. The target data is acquired for each receive coil with a reduced field of view, which results in aliasing, so that each coil produces a k-space representation as shown in FIG. 4, which can be Fourier Transformed into an aliased image as shown in FIG. 5. The two aliased images of FIG. 5 are then unfolded to the full field of view on a pixel by pixel basis using reference data, which records the relative responses (sensitivity profiles) of the receive coils 4 and 5. Reduced field of view imaging imposes a requirement of uniformly spaced samples in the phase-encode direction in k-space. The processing concerned with unfolding is done in the image domain.
It is common practice to acquire the reference data with the subject in the magnetic resonance imaging apparatus. For example, the paper referred to mentions using a body coil as a third receive coil, so that the sensitivity profiles of the coils used to unfold the aliased image, can be derived. This is done by comparing the response of each coil whose sensitivity profile is to be determined with that of the body coil, for magnetic resonance signals received from the body of the patient.
Naturally, this increases the time the patient is in the magnetic resonance imaging apparatus.
Separately acquired reference data has been used to avoid this increased period in the imaging apparatus. For example, a phantom such as a volume of water has been inserted into the imaging machine before the patient is inserted, and the response of the receive coils has been measured, by comparing their outputs with that of a body coil over the volume of water. Alternatively, the response of the receive coils has been calculated theoretically.
In either case, the time the patient spends in the imaging apparatus is reduced, and the patient""s exposure to r.f. excitation radiation is reduced. The disadvantage is that the accuracy of the unfolded image depends critically on correct registration being achieved between the data collected when the patient is within the imaging apparatus, and the previously acquired reference data.
The invention provides apparatus for magnetic resonance imaging, comprising means for exciting magnetic resonant (MR) active nuclei in a region of interest, an array of at least two r.f. receive coils for receiving r.f. signals from the region of interest, means for creating magnetic field gradients in a phase-encode direction for spatially encoding the excited MR active nuclei, the number of phase-encode gradients corresponding to a reduced field of view compared to the region of interest, means for unfolding a representation of the data received by the coil to the array to restore the full field of view using representations of the sensitivity profiles of the individual coils of the array, and means for comparing a plurality of unfolded representations of the data for various modifications to the sensitivity profile representations used, and for selecting an unfolded representation according to a predetermined criterion.
By applying the modifications and comparing the unfolded representations, correct registration can be achieved and the problem associated with pre-acquired reference data solved.
The sensitivity profile representations may be subject to a series of rotational or translational displacements relative to the representation of the data received, or may be modified to simulate different coil loadings. If desired, the sensitivity data for individual coils may be adjusted.
The comparison may be done in the spatial domain but could be carried out in the k-space domain if desired.
The predetermined criterion may be a singularity (singularity extremum) in a function of the intensity probability over a given range of intensities of the unfolded representations in the spatial domain, for example, a maximum of given magnitude in the intensity probability distribution, or a minimum in the summation of the magnitudes of the products of the intensity probability and the log of the intensity probability, over a predetermined range of the intensities. The latter term (the summation) is sometimes referred to as the entropy of the image.